This invention relates to STOL (short takeoff and landing) aircraft and particularly those employing the Coanda effect to rotate the direction of the jet engine thrust and thereby obtain some vertical lift force directly. The jet engines in such STOL aircraft are mounted above and forward of the wing whereby the jet exhaust gases flow over the upper wing and flap surfaces and are rotated thereby.
The characteristics of boundary layers and the Coanda effect are both well known in the art. Various techniques for control of the boundary layer to improve the aerodynamic flow over a wing airfoil have likewise been devised, examples being U.S. Pat. Nos. 2,646,945 and 2,650,781.
The application of the Coanda effect to rotate the direction of the jet exhaust flow and thereby supplement lift is a more contemporary approach. In the typical application, the jet engine is mounted above and forward of the wing with the exhaust directed to flow over the upper wing and flap surfaces. The rearward edge of the wing normally contains a retractable flap assembly. When extended for takeoff and landing the upper surface of the flap's rearward edge is in a near vertical plane. The Coanda effect causes attachment of the jet exhaust first to the upper wing surface and then onto the upper flap surface. The jet exhaust is thereby rotated to produce a vertical component of thrust, without the implementation of any external deflection means.
The fundamental weakness in the application of the Coanda effect to rotate the thrust of the jet exhaust through any significant angle has been the nature of the boundary layer flow to lose velocity, with respect to the wing surface, and precipitate premature separation of the exhaust flow, i.e. before it reaches a near vertical direction. Furthermore, this detrimental characteristic is more prevalent as the air speed or angle of attack increases.